The Observatorio Sismológico y Vulcanológico de Arenal y Miravalles (OSIVAM), belongs to the Office of Seismology and Volcanology (OSV) of the Costa Rican Institute of Electricity (ICE). Formerly, it was called Observatorio Vulcanológico del Arenal (OVA), since it dedicated mainly to Arenal volcano observations, beside other volcanoes monitoring tasks. Nevertheless, due to the installation of the new geothermal plant located on the foot of Miravalles volcano, which started producing geothermoelectricity in 1994, and the Borinquen-Las Pailas geothermal project located at the base of Rincón de la Vieja volcano, ICE advocated to a continuous seismological and volcanological monitoring of these volcanoes as well. Since that, and also to the link between these two new seismological subnetworks around Arenal and Miravalles volcanoes, OVA transformed in OSIVAM.

Arenal volcano is located in the northern part of Costa Rica, halfway between Guanacaste and Central volcanic ranges (Fig. 1). The height of the volcano is 1.6 km above sea level and 1.1 km above the surrounding area. Being at least 7,000 year old volcano, its current volcanic activity has been continuous since July 1968 and is characterized by Strombolian explosions and lava flows, gas ejection, fumarolic activity, and a permanent lava pool since 1974; sometimes there are small pyroclastic flows. Since 1984 there is continuity in the collection of seismic and visual data.

Miravalles volcano is located in the Guanacaste volcanic Range (Fig. 1). The complex volcano edifice lies over a series of stratovolcanoes constructed through the past 2 million years, in the site of the nested Guayabo Caldera, which main ignimbritic events had occurred 1.5 and 0.6 million years ago. Even it has not been active during the last three centuries, it seems to have had Holocene activity. Three geothermal power plants are operating on the southwestern foot of the volcano since 1994 (144 MW).

Rincón de la Vieja is located at the Guanacaste Volcanic Range, with a summit height of 1.9km above sea level. It is a huge active complex stratovolcano whose current activity consists of sporadic phreatic and phreatomagmatic eruptions, explosions with ash plumes, laharic episodes and fumaroles. At present, geothermal potential of this volcano is being investigated by ICE.

Seismological Instrumental Features

ICE installed a seismological network composed by 12-seismic stations and four strong motion devices in the northern part of Costa Rica. The network, which configuration is sketched in Fig. 1, monitors the seismicity around the Arenal and Miravalles volcanoes.

Around Arenal volcano, there are four seismic stations (FOR, CAS, MAC and REY) and two more stations located farther than 8 km (BAL and CHI); additionally, three strong motion equipments are located at Sangregado dam (6.5 km W of volcano, Fig. 2). There are six seismic stations and one accelerograph around Miravalles Geothermal Field. The signals are telemetered to the main station located in Chiripa, where the Observatory facilities (OSIVAM) is located. The stations are equipped with short period, active geophone with 1 Hz natural frequency, a damping factor of 0.707 and an effective sensitivity of 400 Vm-1s-1. The frequency response of the instruments to ground velocity is flat between 1 and 80 Hz. One horizontal component was orientated radially toward the North and the other tangentially. Each remote station consists of a MARS-88/RC with 4 MB buffer RAM, a three-channel digital seismic measurement system, which has a dynamic range of 120 db, covering a wide range of sampling frequencies, a GPS (Gobal Position System) Time Signal Receiver, solar panels, and buffer batteries. The trigger system is controlled by the ratio STA/LTA (Short to Long Term Average of signal) with a pre-event memory of 32 seconds and a post-event memory of 40 seconds.

Since 1975 a network of analog seismographic (helicorders) stations has been operating around Arenal volcano. Currently, only one analog station (called Fortuna: FOR) is still under operation, located 3.7 km East of the summit crater. This station has a short period vertical seismometer (1 Hz) and is telemetered by telephone line to a recording center at ICE in San José. The digital station called also FOR is located right beside this other station. There are also available six (6) portable seismographic stations MQ-800 and 5 digital portable stations.

Field geological observations (observational monitoring) are carried out periodically in order to check the volcano dynamics and to detect any abnormal phenomena. There is as well a periodical mapping of new lava flows and collection of ash samples for grain-size analysis, mass calculations and composition determination.